Thx mmb that is what I was looking for, when dealing with multiple grains do you have to take a weighted sum of PPG in order to calculate the O.G, for example:
Malt % PPG
Pale malt = 86% 37 ppg
Munich = 7% 33 ppg
Crystal 60L = 6% 34 ppg
Total PPG = (.86)*(37) + (.07)*(33) + (.06)*(37) = 36.35
O.G. = (12.75 lbs)*(36.35)*(.75) / 5.5 gal = 63.199
O.G. = 1.063
Checked out on recipator, is a weighted sum the best way to calculate O.G. by hand?